For Teachers Engineering Design in Oregon Science Classrooms Page 1 of 1 “Wait, What Just Happened?” Answers Resource for Calorimeters PART I: HOW DID YOU DO THAT? 1. What do you think caused the water temperature to drop drastically? Any ideas are fine, encourage creative thinking. Actual cause is that the endothermic reaction to dissolve salt draws energy away from the water on the outside of the cup, causing a temperature drop and phase change. 2. Did the reaction in the beaker release heat energy to its surroundings or did it absorb heat energy from its surroundings? Why? It absorbed heat. Energy is required to dissolve potassium chloride in water. Dissolving potassium chloride in water is endothermic because breaking the bonds between the potassium and chloride requires energy and thus absorbs energy. When the resulting potassium and chloride ions form attractions with the water molecules, energy is released but this energy is less than the energy absorbed by breaking the crystal lattice bonds of the potassium chloride so the net result is absorption of energy. In other words the change in enthalpy is positive for potassium chloride. 3. Can you think of a good way to measure the amount of energy absorbed in this reaction? List your ideas below. Especially encourage all creative ideas. Ideally, one would want a calorimeter, an adiabatic device that measures the change in temperature of a substance to determine how much heat was exchanged. 4. Bonus question: Why didn’t the water in the beaker freeze while the water outside of the beaker did? Encourage many ideas. The water inside the beaker did not freeze because the salt lowers the freezing point. Reference elementary chemistry books concerning freezing point depression. PART III: BACKGROUND QUESTIONS 1. What type of cup do you think would be the best for retaining heat? Why? A nice coffee cup, especially foam ones or ceramic mugs. A thermos would also be good. The insulating properties would help keep the heat inside. 2. What materials do you think would improve any cups’ insulating properties? Cloth, foam or certain plastics might improve a cup’s insulating properties 3. Which do you think is a better insulator: aluminum foil or cloth? Cloth 4. Where might a calorimeter lose heat? Air, calorimeter walls, etc. Encourage multiple ideas.